The present invention relates to a multilayer polypropylene film which comprises at least one base layer containing polypropylene and at least one outer layer which contains a mixture of two mixture components.
German Patent Application P 41 35 096.0 discloses multilayer polyolefin films comprising a polypropylene base layer and an outer layer made from an HDPE blend. This HDPE blend comprises HDPE and copolymers and/or terpolymers made from .alpha.-olefins and, if desired, polypropylene. The outer layer of the multilayer film has minimum sheen and maximum haze, giving the film a characteristic matt appearance.
DE-A-16 94 694 discloses multilayer films comprising biaxially-oriented polypropylene films and at least one heat-sealable layer made from an ethylene-propylene copolymer. These films have good heat-sealability, but are clear and have inadequate scratch resistance. In addition, they have unsatisfactory processing properties on high-speed packaging machines.
EP-A-0 008 904 discloses a biaxially-oriented three-layer polyolefin film that is heat-sealable on both sides, in which the base layer is formed from propylene polymers and the two heat-sealable layers are formed from heat-sealable olefin polymers. However, this polyolefin film is transparent and, in particular, has only low scratch resistance and is not printable. In addition, it is also often unsatisfactory with respect to its sliding properties with respect to high-speed packaging machines.
A further biaxially-oriented three-layer polyolefin film that is heat-sealable on both sides is disclosed in U.S. Pat. No. 4,419,411. The two heat-sealable layers in these films incorporate polysiloxane and silicon dioxide as an additive combination. The base layer essentially comprises polypropylene and contains a small amount of a monocarboxamide, which partially migrates from the base layer into the two heat-sealable layers. The multilayer polyolefin film described is said to have a particularly low coefficient of friction. This film has, in particular, the disadvantage that it is not printable.
U.S. Pat. No. 4,578,316 discloses a biaxially-oriented multilayer polyolefin film with a base layer of a polypropylene homopolymer and an outer layer of a blend of polypropylene and MDPE and/or HDPE. This polyolefin film has a low coefficient of friction together with a readily wettable surface; however, this film also has excellent optical clarity.
WO 89/10839 describes multilayer polyolefin films with a base layer of a polypropylene polymer and outer layers of either HDPE or an ethylene-propylene copolymer or a polypropylene, the film being stretched under particular temperature conditions. It has, in particular, a low coefficient of friction and good wettability, but simultaneously also high sheen and low haze.
Thus, the state of the art suggests that various applications require polypropylene films to have various properties and in particular specific combinations of individual quality features.
EP 0 367 613 discloses a multilayer film comprising a vacuole-containing polypropylene base layer and an outer, writable layer that comprises a first polymer having a melt flow index of less than or equal to 1 g/10 min and a second polymer that is incompatible with the first polymer. HDPE having a density of 0.92 to 0.97 g/m.sup.3 is named as an example for the first polymer. As the incompatible polymer, a polypropylene or a copolymer or terpolymer of polypropylene is mentioned. The film has an opaque appearance, i.e., it is essentially impermeable to light. The disclosure states that the film is given a matt appearance by providing it with a print. Preferably, a filler is additionally worked into the outer layer in order to achieve better writability. At the same time, the SiO.sub.2 -containing outer layer shows a high degree of mattness, such that the film is given a paper-like appearance. The film provided with a writing has a density of about 0.69 g/cm.sup.2. Due to its opacity it is, however, unsuitable for a number of purposes. When being printed, the matt, SiO.sub.2 -containing surface is unsatisfactory, for the colors exhibit a grey haze and non-uniform appearance. This is supposed to be due to inhomogeneities, which are not visible on the film alone because of its opacity, but which manifest themselves during printing.
Materials having a silk-matt appearance that have been predominantly employed hitherto are acetate films. However, preparation of these acetate films is associated with considerable environmental pollution. In addition, the stability of these materials in a moist environment is very unsatisfactory. There is therefore an increasing demand for the replacement of acetate films in the existing areas of application, it being desired for the substitute products to come as close as possible to the characteristic silk-matt appearance of acetate films.